This invention relates to metastable silane hydrolyzate solutions and methods for their preparation, and more particularly to the use of such metastable solutions to coat substrates with protective films of insoluble silsesquioxane resins. Such coatings can then be ceramified by heating to form planarizing coatings on substrates.
Condensed hydrogen silsesquioxane resins (HSiO.sub.3/2).sub.n, particularly those soluble resins having a low molecular weight (i.e., where n&gt;8 but total molecular weight is less than 50,000), are known. Hydrogen silsesquioxane resins have found use as protective coatings applied over metal substrates, as primer coatings to increase adhesion of other sealant coatings to a surface, and as cross-linking agents for silicone elastomer resins. More recently, condensed hydrogen silsesquioxane resins have been proposed as ceramic precursors for planarizing layers on electronic devices. See, for example, Haluska et al, U.S. Pat. Nos. 4,753,855 and 4,756,977.
Early methods for the preparation of hydrogen silsesquioxane in hydrocarbon solvents resulted in a resin which was not fully condensed, i.e. the resin contained residual end blocking hydroxyl or alkoxy groups attached to silicon atoms. Such hydrogen silsesquioxane resins were taught to be susceptible to further condensation to form insoluble gels. See Boldebuck, U.S. Pat. No. 2,901,460, for a discussion of hydrolysis reactions of chlorosilanes.
Collins and Frye, U.S. Pat. No. 3,615,272 and J. Amer. Chem. Soc. 92:19 (1970), reported the synthesis of a fully condensed hydrogen silsesquioxane resin, now known to have contained in the range of 140-330 ppm silanol content, which was soluble in nonpolar organic solvents such as benzene and hexane. The Collins and Frye method added trichloro-, trimethoxy-, or triacetoxysilanes in a hydrocarbon solvent to a two phase reaction medium comprising a concentrated sulfuric acid phase and an aromatic hydrocarbon phase such as benzene, to effect condensation of the silanes.
The fully condensed hydrogen silsesquioxane is recovered by washing the reaction mixture with water until neutral and then evaporating the hydrocarbon solvent. However, the process requires the use of both a corrosive acid and an aromatic hydrocarbon in the synthesis. Further, the resin remains soluble even after coating on a substrate, making it undesirable for planarization building up thicker coatings through multiple applications as each successive coating application will redissolve the previously applied coating unless a catalyst or crosslinking agent is added prior to deposition.
Japanese Kokai Patent No. 60-86017 also reports a process for the preparation of a soluble, fully condensed hydrogen silsesquioxane resin by dissolving a trichlorosilane reactant in a water-saturated organic solvent. The trichlorosilane is hydrolyzed and condensed by bubbling an inert gas and water vapor through the reaction solution. It is taught that care must be taken not to form a separate water phase during the reaction as this results in the formation of an insoluble polycondensed gel product. Consensable silanol (Si-OH) groups are end-blocked by the addition of trimethyl chlorosilane as a silylating agent.
Others have reported the formation of siloxane compositions when a chlorosilane starting material is reacted with a metal oxide. For example, Hyde, U.S. Pat. Nos. 2,629,725 and 2,580,852, teaches the reaction of chlorosilanes with metal oxides such as copper, zinc, manganese, and magnesium oxides to form organosiloxanes. The formation of certain cyclic trimers, in the form of substituted cyclotrisiloxanes, by the reaction between chlorosilanes and metal oxides has also been reported. See, Takiguchi et al, J.Org. Chem., 25, 310 (1960) and Wu, U.S. Pat. No. 3,876,677. More recently, Marko et al, U.S. Pat. No. 4,578,494 has taught the reaction of halosilanes in the presence of certain metal oxides and sulfolane to form polysiloxanes.
However, there still remains a need in the art for a relatively simple synthesis procedure which results in the formation of curable silsesquioxane resin solutions and which avoids the use of corrosive acid and/or aromatic hydrocarbon media. Further, the need still exists in the art for silane-containing hydrolyzate which is metastable in solvent solution. Still further, the need exists for a silane-containing hydrolyzates which form an insoluble silsesquioxane resins after coating on a substrate, and which can be repeatedly applied to form relatively thick coatings on substrates.